Hydrogen embrittlement of 1100 aluminum
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H. DEXTER
I n t e r g r a n u l a r f r a c t u r e , with v e r y l i m i t e d p l a s t i c deformation in the g r a i n body, a c c o m p a n i e s b l i s t e r formation in 1100 aluminum exposed to humid a i r in a muffle furnace at 773 K. This c o n t r a s t s with the t r a n s g r a n u l a r f r a c t u r e by m i c r o v o i d c o a l e s c e n c e typical of h y d r o g e n - c h a r g e d aluminum, but is s i m i l a r to o b s e r v a t i o n s 1 of i n t e r g r a n u l a r c r a c k i n g of high strength A1-Zn-Mg a l l o y s exposed under s t r e s s to moist air. B l i s t e r s a r e known to form on s u r f a c e s of a l u m i num heated in water v a p o r , z The vapor r e a c t s with aluminum to form aluminum oxide and atomic h y d r o gen, which is a b s o r b e d . The aluminum l a t t i c e b e comes s u p e r s a t u r a t e d with hydrogen after the r e a c tion, b e c a u s e the effective atomic hydrogen p r e s s u r e during the r e a c t i o n is many t i m e s the equilibrium p r e s s u r e . Subsequent p r e c i p i t a t i o n of m o l e c u l a r h y drogen as " v o i d s " leads to the development of high p r e s s u r e s and b l i s t e r s . The aluminum oxide f o r m e d during the r e a c t i o n r e t a r d s hydrogen evolution, 3 and thus enhances b l i s t e r formation. B l i s t e r s w e r e developed on 2.5 x 10-4 m - t h i c k d i s c s of 1100 aluminum annealed in r o o m a i r in a muffle furnace at 773 K. After this t r e a t m e n t , s e v e r a l of the b l i s t e r s were opened to e s t a b l i s h the topography of the internal s u r f a c e (Fig. 1). The b l i s t e r i n g p r o c e s s was c l e a r l y i n t e r g r a n u l a r f r a c t u r e (Fig. 2).
Although a l a r g e number of FeAl 3 p r e c i p i t a t e s w e r e a p p a r e n t on the grain s u r f a c e s , and holes c o r r e s p o n d ing to s i t e s where p r e c i p i t a t e s w e r e r e m o v e d from adjacent g r a i n s u r f a c e s w e r e often found, the p r e c i p i t a t e s w e r e neither broken nor s u r r o u n d e d by r e g i o n s of l a r g e p l a s t i c s t r a i n . This o b s e r v a t i o n is quite s i m i l a r to r e s u l t s of s t r e s s - c o r r o s i o n t e s t s of high strength aluminum a l l o y s , which show x that " t h e c r a c k does not cut through grain boundary p r e c i p i t a t e s , but
(a)
Fig. 1--Opened and u n d i s t u r b e d b l i s t e r s on the s u r f a c e of 1100 aluminum.
M. R. LOUTHAN, Jr. is Research Supervisor, Materials Research Group, Nuclear Materials Division, and A. H. DEXTER is Research Physicist, Reactor Safety Group, Reactor Engineering Division, Savannah River Laboratory, E. I. du Pont de Nemours and Company, Aiken, SC 29801. Manuscript submitted April 11, 1974. METALLURGICAL TRANSACTIONS A
(b) Fig. 2 - - I n t e r g r a n u l a r f r a c t u r e leading to b l i s t e r f o r m a t i o n (a) b l i s t e r c r o s s - s e c t i o n ; (b) topography of b l i s t e r i n t e r i o r a r r o w s point t
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